A technique for forming thin film transistors using a semiconductor thin film formed over a substrate having an insulating surface has attracted attention. The thin film transistors are used for display devices typified by liquid crystal televisions. A silicon-based semiconductor material is known as a material for a thin semiconductor film applicable to a thin film transistor. As another material, an oxide semiconductor has attracted attention.
As a material for the oxide semiconductor, zinc oxide and a material containing zinc oxide as its component are known. Further, a thin film transistor formed using an amorphous oxide (oxide semiconductor) having an electron carrier concentration of less than 1018/cm3 is disclosed (Patent Documents 1 to 3).